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Boiler Details.

Board of Trade Rule gives half a sq. in. valve area tor every
sq. ft. of grate surface. (See p. 902.)

Mudhole Cover.—Manhole covers are merely flat plates
covering the raised mountings shewn at Figs. 310 and 311 : mud-
hole covers. Fig. 680, are more perfect mechanically, the oval
plate A being kept closed by the steam pressure, and further
secured, by bolts, to bridge pieces B B. The oval shape permits
the plate to be entered narrow-ways, after which it is adjusted
into position.

Pressure and Vacuum Gauges.—The Bourdon Gauge,
Fig. 681, is now generally adopted for both purposes. Within
the casing A is a curved -tube c, of flattened section, as at D : it is
open to pressure at B, but blind at E. If the pressure increase
above the atmosphere, the tube distends, and point E moves out-
ward ; but a decrease of pressure below atmosphere still further
flattens the tube, and point E moves inward Both movements
are transmitted to sector F, which turns, by a pinion, the pointer,
thus multiplying the motion; and a hairspring on the pointer axis
takes up backlash. The graduations are made by experimental
comparison with a mercury gauge.

Injectors.—There are two ways of feeding a boiler with
water when under steam : (i) by a pump either driven from the
•engine, or steam-driven and self-contained, then known as a
f donkey-purnp j* (2) an injector may be used. Pumps will be
treated in the next chapter. The injector forces water into the
boiler without the intervention of moving mechanism, and the
only loss is that due to fluid friction, while the delivery of hot
instead of cold water is a gain, not to speak of the diminished
strain on the boiler. In Fig. 682, A is a section of the instrument,
and B shews its application. The injector represented, being
non-lifting, must be placed at the tank bottom, but its parts are
essentially the same as those of other injectors, c is the water-
cock, D the steam cone, E the combining cone, F the overflow
pipe, and G the delivery pipe. H is the steam cock, and j a
non-return clack box. Cocks H and c being opened, water is
forced into the boiler by the steam; and it long remained a
surprise to engineers that steam could feed water against its own
pressure. The explanation is this: the velocity of efflux of steam